FIG. 35 is a view illustrating an antenna branching filter. In the antenna branching filter 81, a transmitting filter 82 is coupled between a common terminal Ant and a transmitting terminal Tx, and a receiving filter 83 is coupled between the common terminal Ant and a receiving terminal Rx. When the antenna branching filter 81 is used, for example, in a cellular phone, the common terminal Ant is coupled to an antenna, the transmitting terminal Tx is coupled to a transmitting circuit, and the receiving terminal Rx is coupled to a receiving circuit.
The pass band of the transmitting filter 82 is different from that of the receiving filter 83 and thus a transmit signal inputted to the transmitting terminal Tx is, after passing through the transmitting filter 82, outputted via the common terminal Ant to the antenna without flowing in the receiving filter 83. Then, the transmit signal is converted to radio wave and radiated by the antenna. A receive signal received by the antenna passes through the receiving filter 83 and is inputted via the receiving terminal Rx to the receiving circuit. This is an ideal signal flow in the antenna branching filter.
However, in the actual antenna branching filter, all the transmit signals outputted from the transmitting filter 82 are not outputted via the common terminal Ant to the antenna. A part of the signals flows into the receiving filter 83 and reaches the receiving terminal Rx. Consequently, a receive signal inputted via the receiving terminal Rx by the receiving circuit includes noise, thus degrading the receiving performance.
As a value indicating the extent of suppression of the leaking of the transmit signals inputted from the transmitting circuit to the transmitting terminal Tx to the receiving filter 83 to the receiving terminal Rx, transmission-reception isolation (hereinafter, simply referred as “isolation”) is used. Typically, the isolation of an antenna branching filter is approximately 50 dB. The isolation value of 50 dB is, however, not satisfactory for the performance required of an antenna branching filter.
In order to compensate for such receiving performance degradation, a filter is arranged in the receiving circuit. The arrangement increases the size and cost of the apparatus and also makes the apparatus more complex. Consequently, the improvement of isolation in the antenna branching filter has a large effect on cellular phone design.
Thus, Japanese Patent Laid-Open Nos. 2006-60747 and 2002-76829 have discussed an approach for removing unwanted electromagnetic coupling, which is one of the factors causing a transmit signal leakage from the transmitting circuit to the receiving circuit, in an antenna branching filter. According to the Japanese Patent Laid-Open No. 2006-60747, a shield electrode is arranged to remove unwanted electromagnetic coupling. The Japanese Patent Laid-Open No. 2002-76829 discusses an example in which an innovative method of coupling the ground terminal of an antenna branching filter package is used to prevent unwanted electromagnetic coupling.